Two chiral catalysts in action: insights into cooperativity and stereoselectivity in proline and cinchona-thiourea dual organocatalysis

Increasing use of two chiral catalysts in cooperative asymmetric catalysis in recent years raises some fundamental questions on chiral compatibility between the catalysts, modes of activation, and relative disposition of substrates within the chiral environment of the catalysts for effective asymmetric induction. We present molecular insights into a one-pot catalytic Michael reaction cascade between a dicarbonyl compound (7-oxo-7-phenylhept-5-enal) and nitrostyrene, catalyzed by two chiral organocatalysts (proline and cinchona-thiourea), leading to a densely functionalized tetra-substituted cyclohexane product. The density functional theory (SMD(toluene)/M06-2X/6-31G**) computations helped us identify the role of the organocatalytic catalytic dyad in providing a lower energy pathway. The covalent activation of the aldehydic end by (S)-proline results in an enamine, which then adds to the noncovalently activated nitrostyrene in the first Michael addition to give a nitronate anion. The configuration at two of the four chiral centers of the product gets fixed in this step whereas that of the remaining two is determined by intramolecular cyclization between the nitronate and the enone. Important mechanistic features such as (a) a lower energy pathway as compared to a proline-only route for the formation of the syn-enamine and its participation in the first Michael addition and (b) the origin of the preferred prochiral faces in the C-C bond formation are traced to the active involvement of the cinchona-thiourea catalyst in conjunction with proline in each step of the reaction. The true cooperative action by both the catalysts is identified as enabled by a network of hydrogen bonding, and π···π stacking between the aryl ring of the cinchona-thiourea catalyst as well as other noncovalent interactions between the catalysts themselves, and that between the catalysts and substrate.

Organocatalytic Asymmetric Synthesis of Bridged Acetals with Spirooxindole Skeleton

The first highly diastereo- and enantioselective synthesis of bridged O, O-acetals embedded with spirooxindoles has been developed. Dioxindoles and 2-hydroxy cinnamaldehydes were employed as the reaction partners in this method. The desired products were obtained via diaryl prolinol TBS ether catalyzed Michael reaction followed by acetal formation with TFA.

Enantioselective anti-Mannich reaction catalyzed by modularly designed organocatalysts

A highly stereoselective method for achieving the anti-Mannich reaction of aldehydes and ketones with ethyl (4-methoxyphenylimino)acetate was realized using the modularly designed organocatalysts (MDOs) self-assembled from cinchona alkaloid derivatives and (A)-pyrrolidien-3-carboxylic acid in the reaction media. The desired anti-Mannich products were obtained in good to excellent yields (up to 93%), excellent diastereoselectivities (up to 99:1 dr), and good to high enantioselectivities (up to 99% ee).

Stereodivergent Catalysis

This review covers diastereo- and enantiodivergent catalyzed reactions in acyclic and cyclic systems using metal complexes or organocatalysts. Among them, nucleophilic addition to carbon-carbon and carbon-nitrogen double bonds, α-functionalization of carbonyl compounds, allylic substitutions, and ring opening of oxiranes and aziridines are considered. The diastereodivergent synthesis of alkenes from alkynes is also included. Finally, stereodivergent intramolecular and intermolecular cycloadditions and other cyclizations are also reported.

A facile method for the synthesis of fused perhydropyrano[2,3-b]pyrans promoted by Yb(OTf)3

A stereospecific three-component domino reaction between glycals, alkylidene malonate and aldehydes catalyzed by Yb(OTf)3 is described. Multi-substituted cis-fused perhydropyrano[2,3-b]pyran derivatives were obtained with high diastereoselectivity.

A Highly Enantioselective Copper/Phosphoramidite-Thioether-Catalyzed Diastereodivergent 1,3-Dipolar Cycloaddition of Azomethine Ylides and Nitroalkenes

In contrast to the plethora of catalytic systems that enable access to any enantiomers of the chiral products by simply choosing between a pair of enantiomeric or pseudoenantiomeric chiral catalysts, few analogously effective protocols exist for the synthesis of compounds bearing multiple stereogenic centers with full control of the absolute and relative stereochemical configurations. Here, we report the application of our previously developed modular phosphoramidite-thioether ligands for the copper-catalyzed diastereodivergent asymmetric 1,3-dipolar cycloaddition of azomethine ylides and nitroalkenes. Our catalytic system enables wide substrate scope, great stereochemical control, and high reaction efficiency.

Catalyst-controlled diastereoselectivity reversal in the formation of dihydropyrans

In dihydropyran synthesis, use of (S)-3b is crucial for overcoming the inherent trans-selectivity and obtaining unusual cis-adducts with high enantioselectivity.

Asymmetric Organocatalytic One-Pot, Two-Step Sequential Process to Synthesize Chiral Acetal-Containing Polycyclic Derivatives from Cyclic Hemiacetals and Enones

We have developed an efficient one-pot, two-step sequential process to synthesize biologically and synthetically important chiral acetal-containing polycyclic derivatives. This novel protocol had been proved to proceed via Michael-lactolization-oxocarbenium ion ring-closing sequence, which was initiated by a key reactive enamine intermediate and interrupted the previously established reaction pathway of two different enones used in this work, and generated the corresponding cycloadducts with excellent stereoselectivity bearing up to seven continuous stereocenters. Both chiral and racemic starting cyclic hemiacetals worked well in this strategy. The synthetic applications of the obtained polycyclic products have also been demonstrated.

Catalytic Asymmetric Inverse-Electron-Demand Hetero-Diels-Alder Reactions

In this review, the recent developments in catalytic asymmetric inverse-electron-demand hetero-Diels-Alder reaction, which is recognized as one of the most powerful routes to construct highly functionalized and enantioenriched six-membered heterocycles, are described. The article is organized on the basis of different kinds of electron-deficient heterodienes, including α,β-unsaturated ketones/aldehydes, o-benzoquinones, α,β-unsaturated imines, N-aryl imines, o-benzoqinone imides, and other aza-olefins.

Complete diastereodivergence in asymmetric 1,6-addition reactions enabled by minimal modification of a chiral catalyst

Catalytic systems that allow selective generation of any diastereomer of a reaction product bearing multiple stereocentres through minimal modification of a single catalyst scaffold remain elusive, particularly for carbon–carbon bond formations requiring simultaneous control of multiple selectivity factors. Here, we report a catalyst-directed pinpoint inversion of diastereochemical preference in the 1,6-addition of azlactones to δ-aryl dienyl carbonyl compounds with full control over other selectivities preserved. This rigorous diastereodivergence is enabled by the slight structural adjustment of a chiral iminophosphorane catalyst, providing access to all the stereoisomers with high regio-, distereo- and enantioselectivity. The utility of this method is demonstrated in the facile stereodivergent preparation of densely functionalized proline derivatives. The experimental and computational elucidation of the origin of the diastereodivergence is also reported.

For reactions producing numerous chiral centres, accessing all potential stereoisomers is synthetically appealing but difficult. Here, the authors report a stereodivergent 1,6-addition of azlactones where minimal changes to the catalyst structure switches the product selectivity.

Switchable divergent asymmetric synthesis via organocatalysis

A variety of chiral organocatalysts, in addition to synergistic metal-based activation, were discussed in diverse switchable stereoselective reactions.

Metal-free diastereoselective construction of bridged ketal spirooxindoles via a Michael addition-inspired sequence

A TfOH-catalyzed highly diastereoselective Michael addition/ketalization sequence of 3-hydroxyoxindoles and ortho-hydroxychalcones was developed, leading to biologically important bridged ketal spirooxindoles in moderate to excellent yields.

Remote stereocontrolled asymmetric 1,6-addition/1,4-addition cascade reactions between cyclic dienones and 2-indolinethiones

Enantioselective 1,6-addition/1,4-addition cascade reactions between cyclic dienones and 2-indolnethiones have been realized, affording chiral spiro[thiopyranoindole-cyclohexanone] scaffolds facilely and efficiently.

Asymmetric synthesis of bicyclic dihydropyrans via organocatalytic inverse-electron-demand oxo-Diels–Alder reactions of enolizable aliphatic aldehydes

An unprecedented highly enantioselective amine catalyzed oxo-IEDDA reaction of cyclic enones with enolizable aldehydes including aqueous acetaldehyde has been developed.